Solar cell

ABSTRACT

A solar cell (10) is proposed, particularly of silicon, having a semiconductive substrate (1) on one side of which an electrical field is provided by, for example, an MIS contact (1, 2, 3) to cause a separation of charge carriers generated by light energy. The minority charge carriers are drawn into the metal (3) of the MIS contact, whereas the majority charge carriers are discharged via ohmic contact zones (4) arranged on the opposite side of the semiconductive substrate. At least one passivation layer (5) is arranged on the semiconductive substrate between the ohmic contact zones, whereby the recombination velocity of the charge carriers in the area of the ohmic contact zones is considerably reduced.

This is a continuation of application Ser. No. 07/059,264, filed June10, 1987, U.S. Pat. No. 4,828,628.

DESCRIPTION

The invention relates to a solar cell of a semiconductive material suchas silicon, in whose semiconductive substrate charge carriers aregenerated by radiation energy, said charge carriers being separatableand thus dischargable, with an ohmic contact being arranged on thesemiconductive substrate.

The cost of a solar cell module is made up in differing proportions ofthe costs for the solar cell itself and of those for encapsulating andframing the cell array to form modules. Manufacturing the solar cellentails in addition to the actual process costs the cost for thesemiconductive material as a prime factor. For this reason, majorefforts are being undertaken worldwide to develop cheaper semiconductivematerial, with a deterioration of the electrical properties and areduction in the efficiency of the solar cell being the usualconsequences.

A further possibility for cutting the costs of solar cells, particularlywhen single-crystal and polycrystalline silicon is used, consists ofdrastically reducing the thickness of the semiconductive substrate andthus the material expenditure. In addition to this reduction in thesemiconductive material, which has a very considerable effect on theoverall price of the cell, there are further advantages for thin solarcells:

increase in the flexibility of solar cells, thus permitting them to befitted to curved surfaces;

increase of the performance/weight ratio, a particular advantage forouter space applications, but also for terrestrial cells;

increased tolerance to high-energy radiation in outer space;

for example, thin silicon ribbons can be used, the manufacture of whichbecomes more economical as the ribbons become thinner;

reduced heating up of the cell, permitting higher operating voltage,provided the infra-red radiation is not absorbed in the cell;

possibility of structuring the cell to permit exploitation of theradiation striking from the rear surface of the solar cell, thusachieving a considerably higher electrical output.

However, the reduction of the solar cell thickness also involves severeand fundamental problems; if the thickness of the semiconductivesubstrate drops below the respective diffusion length of the minoritycharge carriers, the result of the increased recombination of the chargecarriers on the back surface of the solar cell is a considerabledeterioration of the electrical properties (no-load voltage,short-circuit current) and thus of the efficiency of the solar cell.

To date, this problem has been solved, particularly in outer spacecells, by generating an electrical field on the back surface of thesolar cell. A potential barrier for minority charge carriers is built inat the back surface, so that said carriers cannot penetrate as far asthe ohmic rear-surface contact and recombine there, because an ohmiccontact is distinguished by an extremely high recombination velocity.

The electrical field on the back surface can be generated byincorporating impurity atoms in the rear side by means of diffusion fromthe gas phase, ion implantation, or very frequently by an alloyingprocess. In an n⁺ p silicon solar cell, boron or aluminium, for example,are incorporated into the p-doped silicon substrate to generate a pp⁺junction.

These methods are always expensive high-temperature processes which mayreduce the carrier lifetime in the semiconductor volume in addition tocreating numerous defects at the back surface, greatly reducing theefficiency of the electrical field and so leading to a considerablespread in the solar cell data in large-scale production. In addition,defects occur at the edges of the solar cell, and incomplete alloyingand poor diffusion result from "diffusion pipes". Furthermore, unevenpenetration of the rear-surface metal and precipitation of impuritiescan be detected.

The object of the present invention is to design a solar cell of thetype mentioned at the outset such that the recombination velocity in thearea of the ohmic contact is considerably reduced without the usualprovision of a potential barrier for minority charge carriers, withconsequent simplification and cost-reduction of the manufacturingprocess, with the result that large-scale production of large-areassolar cells, particularly silicon solar cells, is possible.

The object is attained on the one hand by a solar cell characterized inthat the electrical field necessary for charge separation can begenerated on the front of the solar cell, that the ohmic contact on therear surface of the solar cell is in some areas arranged directly on thesemiconductor substrate, the areas being interconnected, and that atleast a first passivation layer is arranged between the areas that doesnot comprise oxide present on the semiconductive substrate.

On the other hand, the object is attained by the electrical fieldnecessary for charge separation being generatable on the rear of thesolar cell, by the semiconductive material being in thickness smaller orequal to the diffusion length of the minority charge carriers, by theohmic contact on the front of the solar cell being arranged in someareas directly on the semiconductive substrate, the areas beinginterconnected, and by there being at least a first passivation layerarranged between the areas that does not comprise oxide present on thesemiconductive substrate.

In the solutions according to the invention, it is therefore proposed,among other measures, that the first passivation layer can be arrangeddirectly on the semiconductor substrate, i.e. that there is no oxidelayer between the semiconductive substrate and the first passivationlayer.

By reducing the area of the ohmic contact, preferably by more than 90%of the semiconductive substrate area, the influence of the said contacton the recombination velocity is reduced accordingly. Of particularsignificance here is that recombination of the charge carriers in thearea between the fields of the ohmic contact is greatly reduced by thepassivation layer, also designated the insulator layer. This is ensuredby a high quality for the passivation layer/semiconductor interface.

In an embodiment of the invention, the electrical field is preferablygenerated by a pn junction or an MIS contact. The MIS contact ispreferably built up of p silicon, the natural silicon oxide layer and/ora silicon oxide layer generated at temperatures below 800° C., andaluminum or magnesium, with the MIS contact possibly extending over theentire rear surface of the solar cell.

Between the semiconductive substrate and the first passivation layer,there can also be a second passivation layer.

Moreover, in a further embodiment of the invention, an amorphous siliconlayer containing hydrogen can be arranged between the semiconductivesubstrate and the first passivation layer, thereby positively changingthe efficiency of the solar cell.

In accordance with further embodiments of the invention, the firstpassivation layer is distinguished in that it contains hydrogen. It cantherefore comprise silicon nitride or silicon oxynitride which ispreferably generatable by deposition from the gas phase in a reactionfurnace in glow discharge, by photo effect or by cathode sputtering, andin addition comprises foreign ions such as alkali ions or aluminum oxideor aluminum oxynitride. The range between 2 and 300 nm is the idealthickness for the first passivation layer.

The solar cells manufactured in accordance with the principles of theinvention, and particularly the ohmic contact areas on the rear surfacewhich do not extend over the entire semiconductive substrate rearsurface, provide the following advantages:

1. Low process expenditure is necessary to make the solar cells. Anenergy-saving and reliable layer deposition process at low temperaturestakes place. The passivation layers can be deposited in a period from 2to 20 minutes at temperatures of less than 600° C.

2. Easy handling is possible, which leads to an increase in the processyield when the cells are extremely thin, because there is less breakage.

3. The long-term stability of the solar cells is increased by goodpassivation of the rear surface and the edge, since the passivationlayer prevents the penetration of foreign atoms from the outside andthus protects both the metal and the semiconductor from damage.

4. A potential barrier is provided on the edge of the solar cell,preventing direct flow of minority charge carriers from the front to therear (short-circuit, reduction of the parallel resistance). This is ofparticular significance in the case of the thin solar cellspredominantly used. The possibility of short-circuiting is alsoprecluded in the event that the metal on the front extends beyond theedge by mistake.

5. In the case of the passivation layers covering the edges of the solarcells, the front-surface contact can be extended to the rear surface andbonded there. Both connections on the rear surface of the solar cellhave the advantage that in addition to the gains in active surface onthe front due to the absence of a bonding pad, the bonding procedure issimplified, thereby increasing the reliability of the bonding process,an advantage which becomes very clear in automatic productionoperations.

6. The passivation layer designated as the insulator layer on the rearsurface permits the emission of heat radiation. This prevents the cellfrom heating up and a higher operating voltage is obtained.

7. The radiation striking the rear of the solar cell is very thoroughlyexploited, thus providing a very effective and low-cost solar cellusable on both sides. While a relatively thick dead layer together witha high surface recombination speed S₀ is obtained during p⁺ diffusion,so that short light wavelengths are very poorly exploited, the low valueof S₀ in the array according to the invention covers all wavelengthswell.

8. The passivation layer also serves as an anti-reflection layer.

9. It is no problem to design a rear-surface reflector in which a metallayer is deposited over the whole area of the passivation layer. On theone hand the heat radiation is reflected off said metal layer and forcedto exit from the front of the solar cell, on the other hand the path ofthe longer-wave radiation, which is however usable for generatingelectron hole pairs, is extended and the short-circuit current therebyincreased.

10. There are no particular requirements for the presence of definitecharges in the passivation layer, thus making the process considerablymore reliable. Unlike the front surface in inversion-layer solar cells,where a strong inversion is necessarily present in the semiconductor dueto corresponding insulator charges, it is irrelevant in the proposalaccording to the invention whether accumulation, depletion or inversiontakes place. A substantial advantage of the principles of the inventionis that even when inversion takes place (in the case of silicon nitrideto p-silicon), the array functions optimally and the minority chargecarriers do not flow predominantly along the conductive inversion layerto the ohmic contact areas and recombine there. It should, however, benoted that the surface recombination velocity during both accumulationand inversion is somewhat lower than in the case of depletion, since inthe first two cases surface states are occupied with charge carriers.The presence of insulator charges is therefore advantageous, in additionto the main condition that low surface state density values must prevailat the insulator/semiconductor interface.

In this connection, it should be mentioned that ohmic rear-surfacecontacts with grid structure are already known, but only in conjunctionwith a whole-area highly doped zone underneath, e.g. a p⁺ zone withp-doped solar cell substrate. Here, the p⁺ zone must not be too thin, asotherwise the dark current proportion from this zone increases as aresult of increased recombination. A reduction of the voltage is theresult. Frequently, a metal or oxide layer is deposited onto the knownp⁺ zone, said layer partly serving to reduce the surface recombinationvelocity and partly to act as a rear surface reflector or as ananti-reflection layer.

Further details, advantages and features of the invention can be foundnot only in the claims, and the features which may be gatheredtherefrom--for themselves and/or in combination--but also in thefollowing description of the drawings indicating preferred embodiments.

FIG. 1 shows capacitance/voltage curves of preferred passivation layers,

FIG. 2 shows a first embodiment of a silicon solar cell according to theinvention having an ohmic contact on the front surface,

FIG. 3 shows a silicon solar cell having an ohmic contact on the rearsurface,

FIG. 4 shows a further embodiment of a silicon solar cell having anohmic contact on the rear surface.

FIG. 1 illustrates the high- and low-frequency capacitance/voltagecurves of the capacitor structure of first and second passivation layersparticularly suited to surface passivation. This involves a layerstructure of p(100) silicon-silicon oxide-Al₂ O₃ -aluminum, with thethickness of the silicon oxide layer being 1.5 nm and that of the Al₂ O₃layer 960 nm. The silicon substrate with (100) orientation is p-doped.Curve a shows the high- and low-frequency capacitance/voltage curve atan Al₂ O₃ deposition temperature of 500° C., curve b that at 290° C. TheAl₂ O₃ layer is made by pyrolysis of the organometallic compoundAl-triisopropylate. The thin silicon oxide layer was generated bythermal oxidation in N₂ /O₂ gas at 510° C. As FIG. 1 shows, a 290° C.Al₂ O₃ deposition temperature resulted in negative flat band voltages(i.e. positive insulator charges) and high surface state densities(D_(it) ˜10¹² cm⁻² eV⁻¹). It is substantially more favorable to depositAl₂ O.sub. 3 at 500° C. In addition to the now negative insulatorcharges, there is now above all a considerably lower surface statedensity D_(it) =8×10¹⁰ cm⁻² eV⁻¹, making the layer structure so preparedvery suitable for low-temperature surface passivation according to theinvention of solar cells.

Even more favorable with respect to surface state densities and thus ofsurface recombination velocities are the conditions for use of plasmasilicon nitride. Unlike Al₂ O₃, the insulator charges here are alwayspositive. Relatively high surface state densities (D_(it) >10¹² cm⁻²eV⁻¹) were measured on p (100) silicon-1.5 nm Si-oxide-100 nmSi-nitride-Al structures at a 200° C. deposition temperature (of NH₃+SiH₄). After annealing at 450° C., however, this value dropped toD_(it) <1×10¹⁰ cm⁻² eV⁻¹. Similarly low D_(it) values are obtained bydeposition of the plasma silicon nitride layers at temperatures between400° C. and 500° C. The positive insulator charge density also dropsfrom Q_(I) /q=6.5×10¹² cm⁻² at 200° C. to Q_(I) /q=1×10¹² cm⁻² attemperatures above 400° C.

At high temperatures exceeding 500° C., the surface state density risesagain while the charges remain almost constant. This makes the plasmasilicon nitride structures particularly suitable for surface passivation(low surface recombination velocity) of solar cells thanks to theirextremely low surface state densities at deposition/annealingtemperatures between 400° C. and 500° C. These favorable characteristicsare due to the hydrogen present in the insulator layers, which saturatesthe dangling bonds of the silicon.

This advantageous low-temperature passivation of semiconductive surfacesnow permits, in accordance with the idea of the invention, effectivepassivation of solar cell surfaces to be achieved in conjunction withohmic contacts, i.e. the insulator layer can, for example, be deposited,after the ohmic aluminum contacts have been made, onto the siliconsurface and above these contacts (simultaneously protecting saidcontacts from corrosion and serious damage).

Passivation with thermal SiO₂ layers on silicon, said layers beingmanufactured at temperatures in excess of 850° C., would, in addition tocausing high-temperature damage to the semiconductor, require a complexphoto-masking step with SiO₂ etching to be able to apply the ohmiccontacts at a later stage. Said contacts would then also have no outsideprotection.

The previously described passivation method in the vicinity of contactsurfaces provided directly on the semiconductive substrate now permitsmanufacture of thin solar cell structures in particular, distinguishedby their simplicity, inexpensive manufacture and effectivity. It is,however, important in all cases to keep the area of the ohmic contacts(very high recombination rate) as small as possible. The embodiments inFIGS. 2 to 4 should make clear further details and advantages of theinvention.

FIG. 2 shows an embodiment worthy of particular attention of a solarcell 10 constructed in accordance with the invention, said cellcomprising a p-doped silicon substrate as the body 1, a thin siliconoxide layer 2, ohmic contacts 4 directly arranged on the silicon body 1,and an MIS (metal insulator semiconductor) contact composed of thep-silicon of silicon body 1, the silicon oxide layer 2 and a metallizedlayer 3 preferably of aluminum or magnesium. The incident lightgenerates electrons and holes in silicon body 1. The electrons (minoritycharge carriers) diffuse toward the rear MIS contact 1, 2, 3 and aredrawn by the electrical field of the said contact into metal 3 aftertunnelling through oxide layer 2. The holes (majority charge carriers)diffuse towards the front and leave the silicon via the ohmic contacts4. An electrical current therefore flows in an outer circuit (frontsurface+pole, rear surface-pole of solar cell 10, (unlike solar cell 20(FIG. 3) and solar cell 30 (FIG. 4))). Since the silicon oxide layer 2of the MIS contact acts as a tunnel insulator for the minority chargecarriers, a thickness of 3 nm should not be exceeded. The boundary ofthe space charge zone of the MIS contact is shown by the dash-dottedline in the drawing. The electrical field prevailing there is forcollection of the minority charge carriers. The ohmic contacts 4 areshown discontinuously applied to the semiconductive substrate, i.e. notcovering the whole area. The ohmic contacts 4 can here form a geometricpattern formed by interconnected stripes, rings, or dots. Passivationlayers now extend between the ohmic contact zones 4, being composed ofthe silicon oxide layer 2 grown naturally or generated at less than 800°C., and of a further insulator layer, which can serve as a passivationlayer and as an anti-reflection layer. The thickness of the top,designated the first, passivation layer 5 can be approx. 80 nm andpreferably comprises aluminum oxide or silicon nitride. The furtherinsulator layer, designated the second passivation layer, in the form ofsilicon oxide layer 2, can be built up differently on the front and rearsurfaces of the silicon substrate 1. It is therefore possible to varythe thickness on the front surface within wide limits, as the layer doesnot act as a tunnel insulator. It is of course also possible to dispensewith layer 2 completely, i.e. in the area of the first passivation layer5--as is also the case in the embodiments of FIGS. 3 and 4.

Both the natural silicon oxide layer and a silicon oxide layer 2,specially prepared, for example, by thermal or other oxidation methods,can be used for the surface passivation effect central to the invention.By natural silicon oxide layer is meant that layer which is alwayspresent on the silicon substrate 1 and is only a few atom ie layersthick.

The structural composition of the silicon oxide layer 2 itself can bechanged on the front surface by subsequent deposition of the firstpassivation layer 5, e.g. silicon nitride. It is therefore possible, forexample, to convert the silicon oxide into silicon oxynitride.

Unlike in conventional solar cells, the charge carriers generated by thelight, predominantly near the front surface, must first diffuse throughthe entire substrate 1 in order to be collected by the MIS contact onthe rear surface. It is important here that the front surface has a lowsurface recombination velocity, as otherwise a major proportion of thecharge carriers would recombine there. The distance between the ohmiccontacts 4, also called contact fingers and characterised by very highrecombination, should be considerably greater than the substratethickness d or the diffusion length of the minority charge carriers, sothat on average the distance to the collecting MIS contact is less thanthat to the ohmic contact area. The distance between the ohmic contactfingers 4 is limited by the series resistance, which is increased byexcessively low ohmic contact areas to the silicon body 1. The optimumdistance is in the range from 1 to 5 mm and the finger width in therange 50-300 μm, thereby achieving a contact surface proportion of lessthan 10%. Cell 10 has, unlike conventional cells where the collectingcontacts are on the front surface, a collection efficiency thatincreases the thinner the semiconductive substrate is and the greaterits diffusion length L is (d≦L). The maximum efficiency is obtained witha thickness d of silicon substrate 1 of approx. 50-80 μm. This permits asaving in expensive semiconductive material and so a considerablycheaper production of the solar cell. A further advantage of this cellis that the rear-surface metal 3 serves as a reflector for large lightwavelengths, leading to an increase in the short-circuit current andlower heating up of the cell during operation (higher open circuitvoltage). Moreover, the entire cross-section of the cell is exploitedfor the current flow, and the minority charge carriers do not have toflow to the contacts in a narrow area of high sheet resistance (e.g. n⁺emitter area in an n⁺ p solar cell) along the surface. A higher fillfactor is therefore programmed. The solar cell 10 is therefore alsoparticularly suitable for concentrated sunlight.

It is also possible to design the MIS rear-surface contact not over thewhole surface, but in grid form, for example, with the intermediate areabetween the MIS contacts being filled with an insulator layer(preferably silicon nitride). The insulator layer must for examplecontain positive charges in the case of p silicon, in order to induce aninversion layer in the silicon along which the minority charge carriers(in the case of p-silicon electrons) can reach the MIS contacts. Twoembodiments are possible here:

(a) The insulator layer can--as is the case with the front surface ofMIS inversion layer solar cell 20 shown in FIG. 3--extend over the MIScontacts, i.e. their deposition/post deposition-treatment temperaturemust be correspondingly low (≦350° C. in the case of Al/SiO_(x) /p-SiMIS contacts), so that the MIS contacts are not irreparably damaged.This is a very simple process from the technological viewpoint, andensures at the same time that the solar cell can also be illuminatedfrom the rear surface.

(b) The insulator layer can however also be broken on the rear surface,with the metal (MIS contact) being located on the thin oxide (˜1 nm-1.5nm) in these finger-shaped openings extending over certain areas (solarcell with light incidence from both sides). The metal can however alsoextend over the entire insulator layer (e.g. thin silicon oxide/siliconnitride), thereby providing an ideal backsurface reflector.

The structures described under (b) with broken insulator layer permitapplication of the insulator layer at higher temperatures (≦600° C.)prior to metallization) and so achieve better interface properties(lower surface state density etc.). The manufacturing process is,however, considerably more expensive due to the use of photo-masking andalignment.

FIGS. 3 and 4 illustrate further embodiments of solar cells 20 and 30that comply with the principles of the invention. The respective ohmiccontact is arranged on the rear surface of solar cell 20 or 30, likewisein certain areas, as already made clear in connection with FIG. 2. Thesolar cell 20 shown in FIG. 3 corresponds to the design principleaccording to the silicon nitride inversion-layer solar cell with MIScontact described in DE-PS 28 40 096. The silicon substrate or body 6thus has a thin silicon oxide layer 7 which corresponds to the oxidelayer 2 on the rear of the solar cell structure shown in FIG. 2,particularly in the area of the MIS contacts. In the area between theMIS contacts, the oxide layer 7 can be varied in thickness within widelimits; what was stated with respect to the oxide layer 2 on the frontof the solar cell structure 10 shown in FIG. 2 is applicable here. Themetals 8, preferably in the form of aluminum or magnesium, of the MIScontacts are arranged on the front of the solar cell 20, preferably instrips. A further insulator layer 9 which can function as anantireflection layer extends over the entire surface of the solar cell,said layer preferably comprising silicon nitride. A high density ofpositive charges comprising the natural charges and those produced byforeign ions must be located in this silicon nitride layer 9, and inducean inversion layer comprising electrons on the surface of the p-dopedsilicon substrate 6. The electrons (minority charge carriers) generatedby the light diffuse towards the front, unlike solar cell 10 (FIG. 2),are accelerated in the electrical field generated by the positiveinsulator charges, and travel along the highly conductive inversionlayer to the MIS contacts 6, 7, 8, through which they leave the siliconbody and pass into an external circuit. The holes diffuse towards therear surface and leave the silicon body 6 via the ohmic contacts 10(front surface-pole, rear surface+pole of solar cell 20 (as is the casein solar cell 30 too))). Passivation of the area between the ohmiccontacts 10 prevents the electrons recombining with the holes there,particularly in the case of light incidence from the rear surface, andso escaping exploitation. The high density of positive charge isindicated by the plus sign in layer 9, and the inversion layer by thebroken line. Furthermore, the dash-dotted line indicates the width ofthe space charge zone. The silicon nitride layer 9 can--unlike thepassivation layer dealt with in the present invention (layer 5 in FIG.2, layer 11 in FIG. 3 and layer 18 in FIG. 4)--only be deposited orannealed at low temperatures (≦300° C.), as otherwise the MIS contactswould be greatly affected.

For that reason, it is not possible in a simple manner to exploit theeffect described in the present invention of reducing the surfacerecombination velocity by applying or annealing passivation layers ontothe front surface at temperatures between 300° C. and 600° C. This canonly be achieved by a more expensive process, for example, byinterrupting the insulator layer 9 on the front surface and providingthe MIS contact in these finger-shaped openings using metal strips 8. Itis thereby possible to apply or anneal the insulator layer here too athigher temperatures (≦600° C.), followed by metal deposition. Unlike theabove case, however, expensive structuring steps are required. The rearsurface of the solar cell according to FIG. 3 comprises ohmic contacts10 in strip form (preferably aluminum) and the double insulator layerbetween said contacts and at the edge, said layer being formed of thethin silicon oxide layer 7 and the insulator layer 11, preferablyaluminum oxide or silicon nitride.

This rear-surface array, on which the present embodiments center,corresponds to the front surface of the solar cell 10 illustrated inFIG. 2, with the remarks made on the oxide layer 2 (corresponding tolayer 7 in FIG. 3) deserving particular mention.

Rear-surface passivation in combination with the ohmic contact area 10represents a process fully compatible with this type of low-temperaturecell (MIS inversion-layer solar cell), since temperature treatmentbetween 400° C. and 600° C. as used for deposition or annealing of therear-surface insulator layer 11 is necessary anyway to generate thetunnel oxide layer 7 on the front surface and to form the ohmic contacts10. These processes naturally precede formation of the MIS contacts 8and deposition of the anti-reflection layer 9 on the front surface. Thiscell type is particularly suited for thin substrates 6 where edgepassivation to prevent a short-circuit between the front and rearsurface usually presents a problem. This is automatically solved in thiscase by the application of passivation layers, i.e. of insulator layers.As can be seen from FIG. 3, a potential barrier forms in thesemiconductor on the top surface of the cell near the edge due to thetwo insulator layers 9 and 11 overlapping, and prevents the chargecarriers on the semiconductive surface flowing to the rear surface atthe edges. The potential barrier results from a very high positivecharge density in the front-surface insulator layer 9 and a negative--orat least greatly reduced positive--charge density in the rear-surfaceand edge insulator layer 11. As FIG. 3 shows, the entire solar cell 20is enclosed by the impermeable insulator layers 9 and 11, and is soprotected from outside influences. The rear-surface passivationintroduced with this invention now enables all the benefits of a thinsolar cell to be achieved, including reduced heating-up effects.

A particular advantage has proved to be that the cell 20 now utilizesvery effectively the radiation striking both surfaces, i.e. also theradiation striking from behind, dispersed or reflected off whitesurfaces. The insulator layer 11 on the rear surface therefore alsoserves as an anti-reflection layer. It is thus possible to increaseconsiderably the output of cell 20. A simple cell type, capable of beingmanufactured at low temperatures, was therefore provided with reduceduse of expensive semiconductive material 6, almost without additionalcost, and with which sunlight could be exploited to a much greaterdegree. Short-wave light in particular is well utilized thanks to thehigher quality of the rear surface. Previous experience shows that thelight striking from the rear is converted to current with an efficiencyonly 10% less than the light striking the front surface. If radiationstriking from the rear is not to be used, a back-surface reflector iseasily provided by applying a metal layer to the rear-surface insulatorlayer 11. In addition, front and rear surfaces can be textured toincrease the light coupled into the cell.

A solar cell 30 in accordance with the principles of the invention isdescribed using FIG. 4. The cell can be a conventional n⁺ p or p⁺ n solrcell. The n⁺ p solar cell 30 illustrated in the embodiment comprises asemiconductive substrate 12, a highly doped surface layer 13 (here n⁺),an ohmic contact 14 in grid form, and a front-surface anti-reflectionlayer 15. Ohmic contacts 16, also not arranged over the entire surfaceand preferably in grid form, are located on the rear surface, as are thepassivation layers characterizing the present invention preferably inthe form of a silicon oxide layer 17 and an insulating layer 18 doublingas an anti-reflection layer. The insulating layer 18 is preferably madeof aluminum oxide or silicon nitride. By the formation of an n⁺ p solarcell 30 in accordance with the invention, a solar cell designed forlight incidence from both sides is provided in a simple manner, with theadditional advantage that the long-wave heat radiation exits from cell30 so that the operating temperature drops (increase in open circuitvoltage). In addition, texturing of the rear surface in particular iseasily possible. In addition to edge passivation by means of layers 17and 18, it is possible to achieve a complete enclosure of cell 30 withimpermeable insulator layer 18 or 15 as a protection. It is furthermorepossible to apply a metal layer as rear-surface reflector to theinsulator layer 18.

This solar cell 30 (FIG. 4) differs from the MIS inversion-layer solarcell 20 (FIG. 3) in that a surface layer highly doped with foreign atoms(e.g. phosphorus) is applied by a high-temperature process to thesilicon body 12, so that an n⁺ p junction and thus an electrical fieldis generated between the layers 12 and 13. The electrons generated bythe light follow the electrical field to the surface, where they passalong the highly conductive n⁺ layer 13 to the ohmic contacts 14 into anexternal circuit. The holes pass to the rear surface and leave thesilicon body 12 via the ohmic contacts 16 (front surface-pole, rearsurface+pole of colar cell 30).

So whereas in the case of solar cell 30 the silicon body practicallycomprises two sections 12 and 13, the solar cells 10 (FIG. 2) and 20(FIG. 3) are homogeneous silicon bodies 1 and 6 respectively, which arenot altered by the manufacturing process of the cell.

We claim:
 1. A solar cell comprising:a substrate layer of semiconductor material of a first conductivity type, in which energy from a radiation source generates minority and majority charge carriers, the substrate layer having a first surface facing the front of the cell and a second surface facing the rear of said cell; a highly doped semiconductor layer of second conductivity type formed on said first surface of said substrate layer, said second conductivity type being opposite to said first conductively type; a plurality of spaced first ohmic contacts formed on said highly doped layer; a plurality of spaced second ohmic contacts formed directly on said second surface of said substrate layer, an insulation, passivation, and antireflection layer formed over and between said second ohmic contacts, said highly doped semiconductor layer of second conductivity type forming junction means for providing an electrical field for separating the minority and majority charge carriers in the region of the first surface of said substrate layer so that the majority charge carriers diffuse to said second ohmic contacts and are collected by them, the thickness of said substrate layer being less than or equal to the diffusion length of the minority charge carriers in said substrate layer.
 2. A solar cell according to claim 1 further comprising a front surface anti-reflection layer formed over said first ohmic contacts.
 3. A solar cell according to claim 1 wherein said first ohmic contacts are arranged in a grid form.
 4. A solar cell according to claim 1 wherein said second ohmic contacts are arranged in a grid form.
 5. A solar cell according to claim 1 wherein said insulation, passivation, and antireflection layer is aluminum oxide.
 6. A solar cell according to claim 1 wherein said insulation, passivation and anti-reflection layer is silicon nitride.
 7. A solar cell according to claim 1 wherein said highly doped layer is doped with phosphorus atoms.
 8. A solar cell according to claim 1 wherein said insulation, passivation, and anti-reflection layer has a thickness of 2-300 nm.
 9. A solar cell according to claim 1 wherein said cell is adapted so that the front of the cell receives radiation from said radiation source.
 10. A solar cell according to claim 1 wherein said cell is adapted so that the rear of the cell receives radiation from said radiation source.
 11. A solar cell according to claim 1 wherein said cell is adapted so that both the front and rear of the cell receive radiation from said radiation source. 